The formation of integrated circuit chips involves many processes. From design to production, each aspect requires considerable attention to details and precision to create and maintain high yield of functioning chips. The metal most commonly used for connecting device parts and interconnecting integrated circuits is aluminum. During the manufacture of the devices, etching aluminum features is required. Specialized aluminum etches are required for various situations found in different approaches to devices and device manufacture to optimize yield and device life. Frequently the ability to selectively etch certain metals (e.g. aluminum) in the presence of other metals is advantageous.
The increasing density of contact terminals for input/output connections in wire bonded devices resulted in overcrowding around the periphery of the device which causes low device yields and early device failures. Solder bumping is one method employed to solve the high-density interconnection problem. In this method, the interconnections are rerouted to various points on the surface of the device not on the periphery of the device.
There are various approaches and materials employed in the production of solder bumps, such as described in U.S. Pat. Nos. 5,508,229, 6,130,141, 6,258,708, 5,902,686, 6,015,652, 5,137,845, 5,773,359, 57,364,456, 4,927,505, 5,904,859, 5,162,257, 5,327,013, and 5,903,058 and U.S. applications Ser. Nos. 09/961037 and 10/003240 herein incorporated by reference.
A solder bump is generally a relatively low melting metal alloy that is placed on a metal pad connected to various circuits. Various materials such as Sn, Pb, Au, Ag, and Cu have been employed as bump components, typically as alloys. A common bump alloy is Sn/Pb alloy.
When making interconnections, the bumps are aligned with the connection points on the device or package to which it is being interconnected and then heated to reflow the solder bumps so that good connections between the circuits are made. The integrity of the solder bump determines the reliability and the chip's functionality.
The metal most commonly used for interconnecting integrated circuits is aluminum. However, techniques for solder bumping directly to aluminum have not been generally successful, so other metals, usually multiple layers of them, are placed between the bump and the aluminum contact pad. These metals are called under bump metals (UBM). These metals are applied to ensure adhesion, provide oxidation protection, wettability to solder, and to provide barrier layers to prevent migration of certain metals. Examples of common under bump metals include Ni, V, Ti, W, Cu, Al, Au, Cr, or their alloys. These metals may be deposited and patterned to cover the metal pad or may be deposited through holes in a removable mask that are over the metal pads. The solder bumps are then applied through a mask, typically through electroplating. The mask is then removed.
Depending on the exact manufacturing techniques employed, Al etching may be done in the presence of the bump and thus also in the presence of the under bump metals such as NiV alloy and Ti. This requires a selective etching process and selective etchant. U.S. applications Ser. Nos. 09/961037 and 10/003240 describe situations in which a selective etchant might be useful.
Aluminum brighteners, cleaners, and etchants typically include phosphoric acid and nitric acid as described in U.S. Pat. Nos. 5,464,500, 6,127,279, 4,230,522, 4,022,930, 4,629,539, 4,857,225, 3,715,250, and 3,645,790. However, when etching aluminum with etchants having phosphoric acid in the presence of the commonly used Sn/Pb solder bumps, there is an undesired side reaction with the bump leading to growth of insoluble phosphate precipitates which may lead to poor device life and shorted circuits unless prevented or removed. An etchant that prevents such precipitates is desired to increase device yields and lifetime.
Numerous etchant blends composed of acids to etch aluminum and other metals are known in the art. U.S. Pat. No. 5,258,093 describes an etchant solution for fabrication of ferroelectric capacitors derived by mixing a water and nitric/phosphoric/acetic acid composition with a platinum etch composition of water and hydrochloric/nitric acid composition. This produced an aqueous four acid blend of nitric/phosphoric/acetic/hydrochloric acid that was high in hydrochloric and nitric acid content. JP 2002208704 described a blend of nitric/phosphoric/acetic/hydrochloric acids for etching molybdenum thin films. U.S. Pat. No. 4,707,191 describes a mixture of aqueous nitric/phosphoric/acetic/hydrochloric acids and a ferric salt for pickling NiMo alloy. JP2003013261 describes a nitric acid/phosphoric acid, organic acid/amine or ammonium base blend composition used for etching aluminum or aluminum alloys. U.S. Pat. No. 4,895,617 describes an aluminum etchant having water and nitric/phosphoric/acetic and an amine oxide surfactant. U.S. Pat. No. 5,300,172 describes an adhesion tape surface protection method while etching plate materials with an etchant composition containing at least one compound selected from a group that contains hydrogen chloride, nitric acid, acetic acid, and phosphoric acid. However, no specific benefits were defined for any acid combinations and only a mixture of hydrofluoric acid and nitric acid was specifically mentioned. JP06041770 discloses a treating method for silicon wafer surfaces that employs a cleaning solution containing at least one of a group of compounds which contains hydrogen chloride, nitric acid, acetic acid, and phosphoric acid and an anionic or nonionic surfactant or a blend of the two types of surfactant. However, no specific benefits were defined for any acid combinations and only binary combinations were employed.
It is an object of this invention to provide an aluminum etchant that does not produce insoluble phosphate residues when etching aluminum in the presence of Sn Pb solder bumps. It is another object of this invention to provide a process of use for the selective etchant.